In optical devices, for example light microscopes or light macroscopes, in which an object to be investigated is illuminated by a light source, it is generally desirable if the brightness of the light source is modifiable so that the illumination conditions can be optimally adapted to the kind of object to be investigated and to the observation method being used (e.g. bright field, polarization, interference contrast, or phase contrast). Light sources used here are preferably incandescent lamps or halogen lamps, since they are obtainable from numerous manufacturers in a wide variety of configurations in terms of output, operating voltage, filament shape, service life, and color temperature.
On the other hand, a defined and uniform color impression is of crucial importance for routine evaluation of microscope images with high throughput. In pathology, for example, the diagnosis is based to a crucial extent on the color impression of microscopic images of tissue sections. In comparison microscopy and comparison macroscopy, an identically colored presentation is indispensable in order for the comparison task to be performed reliably.
A variety of methods exist for modifying or adjusting the color impression in microscopy. On the one hand, for example, for observation through eyepieces and with the use of arc lamps for illumination, the applied lamp current can be increased in order to modify the color temperature. This is disadvantageous, however, in that the service life of the lamp decreases. An associated increase in brightness, which for certain applications can be desired, may need to be compensated for, for example by means of neutral density filters, for other applications in which the color impression of an object is to be modified.
Spectral emission is furthermore constrained by physical laws (Planck's radiation law), so that the spectral distribution of the intensity is modifiable only within specific limits. Increasing the lamp current is also energy-inefficient.
When a camera image is used, it is possible to perform a white balance at the camera. Cameras that are competitive with the human eye in terms of sensitivity in the context of high-throughput analysis of pathology samples are, however, costly. In addition, a camera image and a subsequent check must be carried out, which can critically slow down the workflow.
It is known to introduce variable colored filters into the illumination beam path, both to furnish a color-neutral illumination at different brightnesses, and to modify the color impression. An approach of this kind is proposed, for example, in DE 101 32 360 C1. The furnishing of such filters proves, however, to be costly in terms of manufacture and relatively coarse when establishing a desired color-neutral brightness setting or a desired change in color impression.
It is also known to use particular prism arrangements in the imaging beam path; this procedure is also to be regarded as complex and costly. In comparison microscopy and comparison macroscopy, bifurcated glass fibers are sometimes used in the illumination beam path so that the light of an illumination source can be used to illuminate both observed objects. Bifurcated glass fibers of this kind are, however, complicated to manufacture and correspondingly costly.
When illuminating microscopic samples and imaging them with objectives having a low magnification and large field of view, the problem furthermore often arises that illumination of the sample is not homogeneous or constant, in particular that it decreases toward the edge of the field of view, so that the overall optical impression is inhomogeneous. In the case of a decreasing illumination intensity toward the edge of the field of view, what results, for example, is a correspondingly darker optical impression at the edge of the field of view. The reason for this is the emission characteristic of conventionally available light sources. The light sources preferably used are in turn the incandescent lamps or halogen lamps mentioned above.
The available concepts for correcting inhomogeneous illumination of the field of view are limited conventionally to neutral density filters that are introduced into the respective illumination beams. Neutral density filters of this kind are considered disadvantageous in that they are not modifiable in terms of the distribution of optical density over the field of view. Flexible adaptation to different inhomogeneous illumination situations is thus as a rule not possible with a neutral density filter. It further proves to be disadvantageous that the full luminous intensity of the light source cannot be used, since homogenization requires that a portion of the light be reflected or absorbed. A radial homogenization filter embodied in this fashion is known, for example, from WO 0005606.
In order to adapt the illumination to different objectives it is usual, especially when objectives having a large field of view are used, to furnish by design a second illumination optical system for lower magnifications, as well as a changing mechanism.
This situation proves disadvantageous in particular with incident illumination systems, since here the object represents a component of the illumination optical system. When multiple objectives are furnished, for example on an objective turret, it is thus necessary for provide for each objective a neutral density filter, as well as a changing mechanism that must be synchronized with the objective turret. Approaches of this kind are inflexible and require considerable design outlay. Approaches of this kind are moreover not sustainable for all the relevant parameters, among which may be mentioned here, for example, objective changing, numerical aperture changing, contrast methods, or also centering and focusing within the illumination system, e.g. adjusting, centering, and focusing the field diaphragm.
It is furthermore known, when digital imaging by means of a camera is used, to perform a subsequent digital illumination correction called a “shading correction.” The contrast of a camera image is determined, however, by way of the illumination intensity and the predefined dynamic range of the camera. In many applications it is therefore not possible to compensate by means of a shading correction for a loss of contrast due to poor or inhomogeneous illumination. A shading correction of this kind is furthermore, as already mentioned, usable only when a digital camera is utilized. This method is not available for direct observation of an object through an eyepiece.